The present invention relates to a display control apparatus, an information processing apparatus, and a control method.
In an information processing system (or apparatus), a display device is generally used as a means for realizing an information visual expression function. A CRT display device is most popular as such a display device, as is well known.
In display control in a CRT display device, an operation for writing an image to be displayed in a video memory (to be referred to as a VRAM hereinafter) arranged in an information processing apparatus and an operation for reading out display data from the VRAM are independently performed.
In the above CRT display control, write access of display data to the video memory to update display information and read access for displaying display information are performed independently of each other. For this reason, desired display data can be advantageously written at an arbitrary timing without considering the display timing in the program on the information processing system side.
The number of display dots on a display device in an information processing apparatus such as a personal computer is generally 640.times.400 to 640.times.480 dots. The number of display colors is a maximum of 16 in most display devices.
Along with recent advance in OS's (Operating Systems) and hardware, it is now possible to mount a display board or card on an existing information processing apparatus so as to increase not only the number of display dots but also the number of colors. The display board or card is a so-called graphic accelerator board (card) (to be referred to as a display control board hereinafter). Liquid crystal displays (LCD) have recently received a great deal of attention as substitutes for existing CRT devices because the LCD takes less space.
In the liquid crystal display, the number of colors is generally smaller than that of the CRT. Therefore, image data to be displayed on the LCD must be processed to some extent, and the processing result must be displayed.
For example, the assignee of the present invention has proposed a display using a ferroelectric liquid crystal cell (this display will be referred to as an FLCD hereinafter) as one type of LCD. One of the features of the FLCD lies in that a liquid crystal cell can retain its display state even after removal of an electric field. More specifically, the FLCD has a sufficiently thin liquid crystal cell, and each elongated FLC element in the cell maintains its aligned state even upon removal of the applied electric field. The FLCD using this FLC element has a memory effect for storing display contents due to the bistable properties of the FLC element. The details of the FLC and FLCD are described in U.S. Pat. No. 4,964,699.
The number of colors of the FLCD is 16 at present. However, binarization techniques such as error diffusion can greatly increase the apparent number of display colors.
When a graphic accelerator board for a liquid crystal display such as an FLCD as an output target is taken into consideration, at least a circuit for converting display data into data to be displayed on a liquid crystal display is required.
To check if such a circuit operates properly, the board is connected to an FLCD (this FLCD is, of course, one that has been confirmed as operating properly), a sample image is displayed, and it is determined by observation with the human eye whether any defective portion is present. This operation is time-consuming and requires much labor of those who test the graphic accelerator boards.
To drive an FLCD, since a display image is stored beforehand and displayed, unlike in a CRT or other liquid crystal displays, a time margin is formed in the continuous refresh driving period. As a result, so-called partial rewrite driving for updating only the display state of an updated portion on the display screen independently of the continuous refresh driving can be performed.
In the FLCD, binarization halftone processing is performed to increase the number of pseudo display colors. A typical example is an ED (Error Diffusion) method for maintaining the image qualities of both a natural image and a character image. This ED processing requires continuity of an image in processing because an error occurring in a given pixel is sequentially diffused (distributed) to adjacent pixels.
The following problem is posed when processing using the ED method and the partial rewrite processing is to be simultaneously performed.
More specifically, as described above, according to the ED method, an error occurring in the process propagates like a wave, so that an image as a processing target must be continuous. If some updated portions are present, these portions become discrete in the vertical direction.
To immediately reflect the ED processing result on the FLCD, the transfer rate of the ED processing result must be equal to that of the FLCD. In this case, the partial rewrite position is not fixed, but may be an arbitrary position on the display screen. To cope with this by directly transferring the ED processing result to the FLCD, some technical problems are left unsolved.